Method and apparatus for data transmission based on multiple cell collaboration

ABSTRACT

A method and mobile station are described for reporting channel quality information for multi-cell cooperation. The mobile station receives a channel quality request message for requesting channel quality information from a base station. The channel quality request message indicates that the mobile station is one of a plurality of candidate mobile stations to perform multi-cell cooperation. The mobile station reports the channel quality information to the base station. The channel quality information includes first channel quality information for the base station and second channel quality information for a neighboring base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. application Ser.No. 13/142,729 filed on Jun. 29, 2011, which is the National Phase ofPCT/KR2009/007799 filed on Dec. 24, 2009, which claims priority under 35USC 119(e) to U.S. Provisional Application No. 61/143,170 filed on Jan.8, 2009 and under 35 USC 119(a) to Korean Patent Application No.10-2009-0026114 filed in Republic of Korea, on Mar. 26, 2009. Thecontents of all of these applications are hereby incorporated byreference as fully set forth herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to multi-cell cooperation in a wireless communicationsystem.

2. Discussion of the Related Art

A multiple-input multiple-output (MIMO) system is a system which usesmultiple transmit antennas and multiple receive antennas to improvetransmission/reception efficiency. In theory, the MIMO system hasmaximum channel capacity when using dirty paper coding in which data ofother users is removed in advance by a base station (BS) when the dataacts as interference to each user, and thus interference from otherusers is reduced. However, the dirty paper coding has a problem in thatit is difficult to be implemented in an actual system since not only atransmitter requires much channel information but also computationalcomplexity is high. Various schemes for concurrently allocating spatialresources to a plurality of users have recently been proposed so thatthe principle of the dirty paper coding can be implemented in practice.

An example of selecting a precoding matrix is proposed in per userunitary and rate control (PU2RC) disclosed in a contribution documentR1-060335 “Downlink MIMO for EUTRA” provided by Samsung Electronics Ltd.In this example, each user selects a precoding vector capable ofmaximizing a channel data rate of each user from a plurality ofprecoding matrices, and feeds back an index of the selected precodingvector and a signal to interference plus noise ratio (SINR) to a BS. Onthe basis of information fed back from each user, the BS selects aprecoding matrix and a user.

Research on a multi-cell cooperative system capable of obtaining adiversity gain by using cooperative communication of a plurality of BSshas actively been conducted in recent years. The multi-cell cooperativesystem has been introduced to provide cell coverage extension,throughput enhancement, performance enhancement in a cell edge region,etc.

In a system in which a feedback is limited, a multi-cell cooperativescheme can be divided according to a level of information sharingbetween BSs. That is, the multi-cell cooperative scheme can be dividedinto: a first scheme in which channel information and transmission datainformation are not shared between BSs; a second scheme in which onlychannel information is shared and data information is not shared betweenBSs; a third scheme in which only data information is shared and channelinformation is not shared between BSs; and a fourth scheme in whichchannel information and data information are both shared between BSs. Itis expected that the fourth scheme has best performance in theory, butif all BSs participating in the multi-cell cooperation share data andchannel information of all users, a load of a backhaul network increasesand thus a problem may arise in actual system implementation. Aprecoding method using a scheme of sharing both channel information anddata information between BSs is disclosed in a contribution documentR1-084114 “Per-cell precoding methods for downlink joint processingCoMP” proposed by Electronics and Communications Research Institute(ETRI).

When data information of users belonging to all BSs participating incooperation is shared between BSs in the multi-cell cooperative system,a load of a backhaul network and a buffer size of a BS are increased.This may act as a significant burden in the designing of a wirelesscommunication system that requires high-speed data transmission.

SUMMARY OF THE INVENTION

The present invention provides multi-cell cooperation using sharing ofchannel information without having to share data information.

The present invention also provides a method and apparatus for datatransmission based on multi-cell cooperation.

The present invention also provides a method and apparatus for feedingback reduced channel information for multi-cell cooperation.

In an aspect, a data transmission method performed by a base station(BS) and based on multi-cell cooperation is provided. The methodincludes receiving at least one channel quality indicator (CQI) fromeach mobile station (MS) in a cell, selecting a plurality of candidateMSs on the basis of the received CQI, receiving from each candidate MS aserving precoding matrix index (PMI) for a serving BS, a neighboring PMIfor a neighboring BS, and a CQI obtained from the serving PMI and theneighboring PMI, selecting a transmission MS and a transmission PMI tobe used for the data transmission on the basis of the serving PMI,neighboring PMI, and CQI received from each candidate MS, andtransmitting data to the transmission MS by using a precoding matrixindicated by the transmission PMI.

The method may further include notifying a result of the selection tothe plurality of candidate MSs.

The transmission MS and the transmission PMI may be selected byexchanging a PMI and a CQI with the neighboring BS so that a sum betweena data rate of the serving BS and a data rate of the neighboring BS isthe greatest in the selected MS.

In another aspect, a data transmission method performed by a BS andbased on multi-cell cooperation is provided. The method includesreceiving first channel quality information from each MS in a cell,sharing channel quality information with a neighboring BS on the basisof the first channel quality information and selecting a transmissionPMI and a neighboring transmission PMI used by the neighboring BS,transmitting the neighboring transmission PMI to each MS, receiving,from each MS, second channel quality information obtained from theneighboring transmission PMI, selecting a transmission MS on the basisof the second channel quality information, and transmitting data to thetransmission MS by using a precoding matrix indicated by thetransmission PMI.

The first channel quality information may include a PMI belonging to acodebook used by the BS and a CQI corresponding to the PMI.

The method may further include transmitting the transmission PMI to eachMS.

In another aspect, a channel information feedback method performed by anMS and based on multi-cell cooperation is provided. The method includestransmitting, to a BS, first channel quality information includingchannel information between the MS and the BS, receiving a channelquality information request message from the BS after transmitting thefirst channel quality information, and transmitting second channelquality information including channel information between the MS and theBS and a neighboring BS in response to the channel quality informationrequest message.

The first channel quality information may include a PMI selected from acodebook and a CQI corresponding to the PMI.

The channel quality information request message may include atransmission PMI of the neighboring BS.

A feedback from a mobile station can be reduced for implementation ofmulti-cell cooperation and a load of a backhaul network can also bereduced. Therefore, a wireless communication system that requireshigh-speed data processing can be implemented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows precoding matrix compositions when G=4.

FIG. 3 is a flowchart showing a data transmission method according to anembodiment of the present invention.

FIG. 4 shows an example of a data transmission method according to anembodiment of the present invention.

FIG. 5 to FIG. 7 show exemplary implementations of a data transmissionmethod.

FIG. 8 and FIG. 9 are graphs showing a simulation result.

FIG. 10 is a flowchart showing a data transmission method according toanother embodiment of the present invention.

FIG. 11 is a block diagram of a base station and a mobile station forimplementing an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Methods, apparatuses, and technologies described below are applicable tovarious radio access schemes such as code division multiple access(CDMA), frequency division multiple access (FDMA), time divisionmultiple access (TDMA), orthogonal frequency division multiple access(OFDMA), single carrier frequency division multiple access (SC-FDMA),etc. The radio access scheme can be implemented with various radiocommunication standard systems. Wideband CDMA (WCDMA) can be implementedwith a radio technique such as universal terrestrial radio accessnetwork (UTRAN) proposed by the 3rd generation partnership project(3GPP) standard organization. CDMA2000 is a radio technology based onCDMA. High rate packet data (HRPD) based on the 3rd generationpartnership project 2 (3GPP2) standard organization provides a highpacket data service in a CDMA2000-based system. Evolved HRPD (eHRPD) isan evolution of the HRPD. The TDMA can be implemented with a radiotechnology such as global system for mobile communications (GSM)/generalpacket ratio service (GPRS)/enhanced data rate for GSM evolution (EDGE).The OFDMA can be implemented with a radio technology such as instituteof electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), etc. Long termevolution (LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA,and uses the OFDMA in a downlink and uses the SC-FDMA in an uplink.LTE-advance (LTE-A) is an evolution of the LTE.

The technology described below may apply to a multiple antenna system ora multiple-input multiple-output (MIMO) system using multiple transmit(Tx) antennas and at least one receive (Rx) antenna. The technologydescribed below may apply to various MIMO schemes. The MIMO schemeincludes spatial diversity in which the same stream is transmitted tomultiple layers and spatial multiplexing in which multiple streams aretransmitted to multiple layers. When the multiple streams aretransmitted to a single user in the spatial multiplexing, it is calledsingle user-MIMO (SU-MIMO) or spatial division multiple access (SDMA).When the multiple streams are transmitted to multiple users in thespatial multiplexing, it is called multi user-MIMO (MU-MIMO). Accordingto whether feedback information reported from each user is used or not,the spatial diversity and the spatial multiplexing can be classifiedinto an open-loop scheme and a closed-loop scheme.

FIG. 1 shows a wireless communication system. The wireless communicationsystem includes one or more base stations (BSs) 10A and 10B. The BSs 10Aand 10B provide communication services to specific geographical regions30A and 30B (generally referred to as cells), respectively. The cell canbe divided into a plurality of regions (referred to as sectors). One BSmay include one or more cells.

Mobile stations (MSs) 20A and 20B may be fixed or mobile, and may bereferred to as another terminology, such as a user equipment (UE), auser terminal (UT), a subscriber station (SS), a wireless device, apersonal digital assistant (PDA), a wireless modem, a handheld device,an access terminal (AT), etc. The BSs 10A and 10B are generally fixedstations that communicate with the MSs 20A and 20B and may be referredto as another terminology, such as an evolved node-B (eNB), a basetransceiver system (BTS), an access point, an access network (AN), etc.

Hereinafter, a downlink (DL) denotes a communication link from the BS tothe MS, and an uplink (UL) denotes a communication link from the MS tothe BS. In the DL, a transmitter may be a part of the BS, and a receivermay be a part of the MS. In the UL, the transmitter may be a part of theMS, and the receiver may be a part of the BS.

The MS 20A located inside the cell 30A communicates with the serving BS10A. A serving BS implies a BS accessed by the MS 20A, and a servingcell implies a cell managed by the serving BS. A neighboring cellimplies a cell located near the serving cell. The neighboring cell isnot limited to a geographical location. The neighboring cell is a cellthat is cooperative to multi-cell cooperation.

Hereinafter, multi-cell cooperation will be described. It is assumedthat one cell exists in one BS, and there are two types of cells, i.e.,one serving cell and one neighboring cell.

Codebook-based unitary beamforming is considered in each cell. Thecodebook includes a plurality of precoding matrices. A serving BS and aneighboring BS may use identical or different codebooks. It is assumedthat the serving BS and the neighboring BS each use Nt Tx antennas, andthe two BSs each transmit L data streams (where L Nt). This implies thatthe serving BS and the neighboring BS have the same rank L. An Rx signalcan be expressed by Equation 1 below.Y _(k,1) =H _(k,1) W _(1,i) S ₁ +H _(k,2) W _(2,j) S ₂ +N_(k)  [Equation 1]

In Equation 1, Y_(k,i) denotes an Nr×1 Rx signal vector of a k^(th) MSof an i^(th) cell, H_(k,i) denotes an Nr×Nt channel matrix between thek^(th) MS and an i^(th) BS, W_(i, g) denotes a g^(th) Nt×L unitaryprecoding matrix of the i^(th) BS, N_(k) denotes an Nr×1 noise vector,and S_(i) denotes an L×1 Tx symbol vector. Herein, Nr denotes the numberof Rx antennas. The rank L denotes the number of data streams that canbe transmitted concurrently, and a maximum value of the rank L is aminimum value between Nr and Nt. For example, if Nt=4, Nr=2, then themaximum rank may be 2.

It is assumed that each BS has a codebook including G unitary matrices.W_(i,s) is a precoding matrix selected from a codebook [W₁, W₂, . . . ,W_(G)]. A precoding matrix index (PMI) is an index of a precoding matrixselected from the codebook. If it is assumed herein that a minimum meansquared error (MMSE) receiver is used, an Rx SINR of an i^(th) streamfor a g^(th) codebook of a k^(th) MS of a 1^(st) cell can be expressedby Equation 2 below.

$\begin{matrix}{{SINR}_{k,1,g,i} = {h_{{eff},1,g,i}^{H} \cdot \left( {{\sum\limits_{j \neq i}^{L}\;{h_{{eff},1,g,j}h_{{eff},1,g,j}^{H}}} + {\sum\limits_{j = 1}^{L}\;{h_{{eff},2,g^{\prime},j}h_{{eff},2,g^{\prime},j}^{H}}} + {\sigma^{2}I_{L}}} \right)^{- 1} \cdot h_{{eff},1,g,i}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Equation 2, h_(eff,i,g,j) denotes an effective channel for a j^(th)stream of a g^(th) precoding matrix for an i^(th) BS of a k^(th) MS, andcan be expressed by Equation 3 below.h _(eff,1,g,i) =H _(k,1) w _(g,i)  [Equation 3]

In Equation 3, w_(g,i) denotes an i^(th) column vector of a g^(th)precoding matrix.

If L<Nt in the codebook-based system, it can be seen that the Rx SINRhas an effect on selection of a precoding matrix of a neighboring cell.This can be proved as follows. A part corresponding to interference ofthe neighboring cell can be expressed by Equation 4 below.

$\begin{matrix}\begin{matrix}{{\sum\limits_{j = 1}^{L}\;{h_{{eff},2,g^{\prime},j}h_{{eff},2,g^{\prime},j}^{H}}} = {\sum\limits_{j = 1}^{L}\;{H_{k,2}w_{g^{\prime},j}w_{g^{\prime},j}^{H}H_{k,2}^{H}}}} \\{= {{H_{k,2}\left( {\sum\limits_{j = 1}^{L}\;{w_{g^{\prime},j}w_{g^{\prime},j}^{H}}} \right)}H_{k,2}^{H}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In Equation 4, if L=Nt and Wg′ is a unitary matrix, then

${\sum\limits_{j = 1}^{Nt}\;{w_{g^{\prime},j}w_{g^{\prime},j}^{H}}} = {I_{Nt}.}$

Herein, I_(Nt) denotes an identify matrix. Thus, irrespective of theprecoding matrix of the neighboring cell, it can be seen that power ofan interference signal is identical to H_(k,2)H^(H) _(k,2). However,since

${\sum\limits_{j = 1}^{Nt}\;{w_{g^{\prime},j}w_{g^{\prime},j}^{H}}} \neq I_{Nt}$when L<Nt, it can be seen that interference power caused by theneighboring cell is influenced by the precoding matrix of theneighboring cell. This implies that if the rank is less than the numberof Tx antennas, a gain can be obtained by the precoding matrix of theneighboring cell.

Therefore, if the number Nt of the Tx antennas is greater than thenumber L of data streams, a precoding matrix of each cell has an effecton an SINR of the neighboring cell, and thus when selecting a precodingmatrix, each BS needs to consider the precoding matrix of theneighboring cell. A scheduling metric can be configured to select aprecoding matrix of a serving BS and a precoding matrix of a neighboringBS. The scheduling metric is a criterion for selecting a user and/or aprecoding matrix for data transmission. A sum of data rates,proportional fair (PR) scheduling, etc., can be used as the schedulingmetric. For example, if the scheduling metric is the sum of data rates,the precoding matrix can be selected so that the sum of data rates ofrespective BSs becomes a maximum value. This can be expressed byEquation 5 below.

$\begin{matrix}{{\max\limits_{W_{1,j},{w_{2,j} \in {codebook}}}\mspace{14mu} R_{1}} + R_{2}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In Equation 5, R_(i) denotes a data rate of an i^(th) BS. When two cellsare considered, precoding matrices W_(1,j) and W_(2,j) are preferablyselected so that a sum (i.e., R₁+R₂) of a data rate R₁ of a 1^(st) BSand a data rate R₂ of a 2^(nd) BS becomes a maximum value.

In order to calculate Equation 5 above, it is necessary for each MS tocalculate an SINR for all possible cases of precoding matrixcompositions by considering selection of the precoding matrix of theneighboring cell and to feed back all of the calculated SINRs. Thisimplies that even if there are only two cells, the number of precodingmatrices to be considered in practice is G². FIG. 2 shows precodingmatrix compositions when G=4. Computational complexity may increase ifeach user calculates all of the precoding matrix compositions. Inadditions, signaling overhead may be caused if SINR information for eachprecoding matrix is entirely fed back to a BS. Therefore, there is aneed for a method capable of decreasing overhead caused by a feedback ofchannel information.

FIG. 3 is a flowchart showing a data transmission method according to anembodiment of the present invention. This method can be performed by aBS. In step S310, the BS selects at least one candidate MS from aplurality of MSs in a cell. By considering not only a codebook of aserving BS but also a codebook of a neighboring BS, each MS sendschannel quality information to the serving BS. The channel qualityinformation includes a selected precoding matrix index (PMI), a rankindicator (RI) for the selected PMI, and/or a channel quality indicator(CQI) corresponding to the selected PMI. The channel quality informationmay further include a PMI for the neighboring BS. The selected PMI maybe a PMI having a best CQI. The CQI can be expressed in various formatssuch as an SINR or a modulation and coding scheme (MCS) index. The BSselects a candidate MS on the basis of the received channel qualityinformation of each MS.

In step S320, the BS collects channel quality information for each PMIbelonging to a codebook from the selected candidate MS. In step S330,the BS selects a transmission MS for transmitting data according to aspecific criterion on the basis of the per-channel channel qualityinformation received from the candidate MSs, and selects a transmissionPMI for the transmission MS. In step S340, the BS transmits the data tothe transmission MS by using a precoding matrix indicated by thetransmission PMI.

The MS can calculate a CQI for G PMIs for the serving BS and G PMIcompositions for the neighboring BS, i.e., G² PMI compositions. In thiscase, when G² CQI values calculated in each MS are fed back to theserving BS, it may be act as a significant overhead. Therefore, insteadof transmitting CQI values for all PMIs to all MS in a cell, a pluralityof candidate MSs are selected from the all MSs in the cell and CQIinformation for the codebook of the serving BS and the codebook of theneighboring BS is selected from the candidate MSs. Then, each BS selectsthe transmission PMI and the transmission MS by using informationcollected from the plurality of candidate MSs, and transmits data byusing the selected PMI and MS. By selecting some of candidate MSs amongthe MSs in the cell, an amount of feedback transmitted to the BS can bedecreased according to the number of selected candidate MSs over thetotal number of MS.

FIG. 4 shows an example of a data transmission method according to anembodiment of the present invention. A 1^(st) BS 40A is a serving BS forK MSs MS1-1, MS1-2, . . . , MS1-K, and a 2^(nd) BS 40B is a serving BSfor K MSs MS2-1, MS2-2, . . . , MS2-K. A neighboring BS of the 1^(st) BS40A is the 2^(nd) BS 40B, and a neighboring BS of the 2^(nd) BS 40B isthe 1^(st) BS 40A. Although the serving BS has one neighboring BS, and KMSs belong to each BS, this is for exemplary purposes only. Thus, thenumber of neighboring BSs and the number of MSs belonging to each BS arenot limited thereto.

In step S410, the MS calculates a CQI on the basis of a PMI compositionfor each rank. CQI values for compositions of all PMIs belonging to acodebook of the serving BS (hereinafter, a serving codebook) and acodebook of the neighboring BS (hereinafter, a neighboring codebook) arecalculated. In each MS, information on the serving codebook andneighboring codebook for each rank may be stored in a memory.Alternatively, the serving BS may report the serving codebook andneighboring codebook to be used by the MS. It is assumed herein thateach of the serving codebook and the neighboring codebook is configuredwith G PMIs. The number of CQIs that can be calculated by the MS maycorrespond to G².

In step S420, the MS feeds back a best CQI, a PMI corresponding to theCQI, and a neighboring PMI. The PMI may include a PMI selected from theserving codebook used by the serving BS (such a PMI is called a servingPMI) and/or a PMI selected from the neighboring codebook used by theneighboring BS (such a PMI is called a neighboring PMI). When two ormore ranks can be configured, the MS can feed back a serving PMI,neighboring PMI, and RI selected for each rank and a CQI correspondingthereto. The MS selects a CQI indicating good channel quality among thecalculated G² CQIs, and transmits the selected CQI, RI, and PMI. It isassumed herein that the number Nr of Rx antennas is 1 (i.e., rank L is 1and thus only one Rx antenna exists), and one CQI is fed back. However,this is for exemplary purposes only, and thus the MS can feed back atleast one CQI and/or PMI for each rank.

Alternatively, the BS can determine a threshold for sending a CQI to theMS. The MS can feed back at least one CQI exceeding the threshold amongthe calculated CQIs. The threshold can be reported by the BS to the MSthrough a broadcast channel and/or a dedicated channel. For example, theBS may report the threshold to the MS as a part of system informationwhich is broadcast information.

In step S430, when one CQI is received from each MS, the BS selects Bcandidate MSs (where B≧1) on the basis of the received CQI, and notifiesit to the selected candidate MSs. The BS can select the candidate MSs ina descending order of the best CQI. Although the 1^(st) BS 40A and the2^(nd) BS 40B equally select B candidate MSs herein, this is forexemplary purposes only. To notify the selection result to the candidateMSs, signaling can be used variously. It can be reported by using abroadcast message such as system information, a radio resource control(RRC) message, or a message to be transmitted through the dedicatedchannel.

In step S440, the selected B candidate MSs transmit (Q-1) CQIs and PMIscorresponding thereto to the BS. In this case, the transmitted PMI mayinclude a serving PMI and/or a neighboring PMI. Herein, Q denotes thetotal number of CQIs transmitted by the candidate MSs to the BS. Sinceone CQI has already been sent in step S410, (Q-1) CQIs are sent in stepS440. A maximum value of Q may be G², and may be reported to the MS whenthe BS notifies to the MS that the MS is selected as the candidate MS,or may be reported by using a separate message.

In step S450, the BS selects one or more MSs having the best CQI on thebasis of the CQI and PMI received from each candidate MS. In step S460,the BS calculates a data rate for each PMI with respect to each of theselected MSs. In step S470, the 1^(st) BS 40A and the 2^(nd) BS 40Bexchange the PMI and the CQI of the selected MS, and selects atransmission MS according to a scheduling metric. A sum of data rates,proportional fair (PR) scheduling, etc., can be used as the schedulingmetric. For example, if the scheduling metric is the sum of data rates,an MS of which a sum between a data rate of the serving BS and a datarate of the neighboring BS is the best can be selected as thetransmission MS. In addition, each BS can select a transmission PMI usedby the transmission MS. Each BS can select one transmission MS, or canselect a plurality of MSs in a magnitude order of the sum of data rates.In step S480, the BS transmits data by using a precoding matrixindicated by the transmission PMI to the selected transmission MS.

Instead of requesting a CQI for all PMI compositions (e.g., G²compositions) from K MSs belonging to each cell, the CQI is requested byselecting B candidate MSs of which a CQI is regarded as relatively good.Herein, B<K. In this case, instead of requesting the CQI for all casesfrom the B candidate MSs, Q CQIs are requested, where Q≦G². That is, itis not necessary for all MSs to feed back all CQIs for the G² PMIs.Rather, all MSs feed back only one CQI, and then based on informationthereof, request (Q-1) CQIs from top B candidate MSs, therebyeffectively decreasing an amount of feedback.

FIG. 5 to FIG. 7 show exemplary implementations of a data transmissionmethod. In FIG. 5, K=10. A 1^(st) BS and a 2^(nd) BS receive one CQI andits corresponding PMI from 10 MSs. The PMI may include a serving PMIand/or a neighboring PMI. In FIG. 6, B=5. A 1^(st) BS and a 2^(nd) BSreceive a CQI based on each PMI composition from 5 selected candidateMSs. In FIG. 7, three transmission MSs are selected. A 1^(st) BS and a2^(nd) BS each transmit data to the selected transmission MSs.

FIGS. 8 and 9 are graphs showing a simulation result. FIG. 8 is a graphfor comparing sum rates when Nr=4, Nr=1, G=4, L=1. Table 1 below showsvalues K, B, and Q, where K denotes the number of MSs in each cell, Bdenotes the number of candidate MSs, and Q denotes the number of CQIsreceived from the candidate MSs.

TABLE 1 K 4 8 16 32 64 128 B 4 6 8 8 9 9 Q 4 3 2 2 2 2

In this case, a simulation is performed for cases where asignal-to-noise ratio (SNR) is 0 DB and 10 dB, and an average of datarates are used by measuring the data rates of finally selectedtransmission MSs. Referring to FIG. 8, a proposed method (i.e., case B)shows a data rate almost similar to that of a case A where a CQI istransmitted for all possible codebooks that can be combined from all MSsin a cell. Further, the proposed method (i.e., case B) shows that a datarate is significantly improved in comparison with a case C where a BScooperation scheme is not used.

FIG. 9 is a graph for comparing sum rates when Nr=4, Nr=1, G=4, L=2.Table 2 below shows values K, B, and Q, where K denotes the number ofMSs in each cell, B denotes the number of candidate MSs, and Q denotesthe number of CQIs received from the candidate MSs. A test result on adata rate is shown in the graph.

TABLE 2 K 4 8 16 32 64 128 B 4 6 13 13 13 25 Q 10 8 4 4 4 2

A proposed method (i.e., case B) shows a data rate almost similar tothat of a case A where a CQI is transmitted for all possible codebooksthat can be combined from all MSs in a cell. Further, the proposedmethod (i.e., case B) shows that a data rate is significantly improvedin comparison with a case C where a BS cooperation scheme is not used.

Although it is shown herein that the number of BSs participating incooperation is 2, the present invention is not limited thereto. Thetechnical features of the present invention can be easily used for nneighboring BSs (where n≧1) by those ordinary skilled in the art.

The BSs participating in cooperation may have independent ranks That is,each BS may have a different rank, and the number of precoding matricesincluded in a codebook may also differ. Rank information on the BSsparticipating in cooperation may be mutually exchanged in advance.

FIG. 10 is a flowchart showing a data transmission method according toanother embodiment of the present invention. In step S1010, each MScalculates a CQI for G PMIs belonging to its serving codebook. That is,unlike the embodiment of FIG. 4, the MS calculates only a CQI for a PMIbelonging to a serving codebook without considering an influence of aneighboring BS. In step S1020, the MS feeds back 1^(st) channel qualityinformation to a serving BS. The 1^(st) channel quality information mayinclude the best CQI among G CQIs, its corresponding RI, and/or PMI. Itis assumed herein that the 1^(st) channel quality information is onerank, and thus includes one CQI and its corresponding PMI.

Alternatively, the BS can determine a threshold for sending a CQI to theMS. The MS can feed back at least one CQI exceeding the threshold amongthe calculated CQIs. The threshold can be reported by the BS to the MSthrough a broadcast channel and/or a dedicated channel. For example, theBS may report the threshold to the MS as a part of system informationwhich is broadcast information.

In step S1030, the BS calculates a data rate for each PMI. In stepS1040, each BS exchanges the PMI and the data rate with its neighboringBS, and selects a PMI having the greatest sum rate. Each BS selects aPMI used by itself (such a PMI is called a transmission PMI) and a PMIused by the neighboring BS (such a PMI is called a neighboringtransmission PMI). Although the sum rate is considered as a schedulingmetric for selecting the transmission PMI and the neighboringtransmission PMI, this is for exemplary purposes only.

In step S1050, the BS broadcasts the transmission PMI and theneighboring transmission PMI. That is, the BS reports the transmissionPMI and the neighboring transmission PMI to all MSs in a cell. Thetransmission PMI and/or the neighboring transmission PMI can betransmitted to the MS by being included in a channel quality informationrequest message. The channel quality information request message is amessage used by the BS to request the MS to measure channel qualitybased on the transmission PMI and the neighboring transmission PMI andto report a result value of the measurement.

In step S1060, the MSs in the cell calculate a CQI on the basis of thetransmission PMI and the neighboring transmission PMI. In step S1070,the MSs in the cell feed back 2^(nd) channel quality information. The2^(nd) channel quality information includes a CQI calculated based onthe transmission PMI and the neighboring transmission PMI. In stepS1080, the BS selects an MS for data transmission on the basis of the2^(nd) channel quality information, and transmits the data to theselected MS by using the transmission PMI and the neighboringtransmission PMI.

FIG. 11 is a block diagram of a BS and an MS for implementing anembodiment of the present invention. A BS 1100 includes a processor 1110and a transceiver 1120. The transceiver 1120 transmits and/or receives aradio signal. The processor 1110 is coupled to the transceiver 1120, andimplements the method described in the embodiment of FIG. 4 and/or FIG.10.

An MS 1200 includes a processor 1210, a transceiver 1220, and a userinterface 1230. The transceiver 1220 transmits and/or receives a radiosignal. The processor 1210 is coupled to the transceiver 1220, andimplements the method described in the embodiment of FIG. 4 and/or FIG.10. The user interface 1230 is coupled to the processor 1210, andprovides an interface with respect to a user. The user interface 1230may include an input tool such as a well-known keypad and/or a displaydevice for providing a user environment.

The present invention can be implemented with hardware, software, orcombination thereof. In hardware implementation, the present inventioncan be implemented with one of an application specific integratedcircuit (ASIC), a digital signal processor (DSP), a programmable logicdevice (PLD), a field programmable gate array (FPGA), a processor, acontroller, a microprocessor, other electronic units, and combinationthereof, which are designed to perform the aforementioned functions. Insoftware implementation, the present invention can be implemented with amodule for performing the aforementioned functions. Software is storablein a memory unit and executed by the processor. Various means widelyknown to those skilled in the art can be used as the memory unit or theprocessor.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

What is claimed is:
 1. A method for reporting channel qualityinformation for multi-cell cooperation, the method performed by a mobilestation and comprising: receiving, by the mobile station, precodingmatrix indicator (PMI) information from a first base station, the PMIinformation including G PMIs available to be used by the first basestation to send downlink data and G PMIs available to be used by asecond base station to send downlink data; receiving, by the mobilestation, a channel quality request message for requesting channelquality information from the first base station, the channel qualityrequest message indicating that the mobile station is selected, by thefirst base station from among a plurality of candidate mobile stations,to perform multi-cell cooperation; calculating G² channel qualityindicators (CQIs) based on G² combinations of the G PMIs of the firstbase station and the G PMIs of the second base station; and reporting,by the mobile station, the channel quality information to the first basestation, wherein the channel quality information includes a first bestCQI and a first PMI corresponding to the first best CQI for the firstbase station, and a second best CQI and a second PMI corresponding tothe second best CQI for the second base station.
 2. The method of claim1, wherein the plurality of candidate mobile stations each reports itschannel quality information to the first base station.
 3. A mobilestation for reporting channel quality information for multi-cellcooperation, the mobile station comprising: a transceiver configured totransmit and receive radio signals; and a processor operatively coupledwith the transceiver and configured to: instruct the transceiver toreceive precoding matrix indicator (PMI) information from a first basestation, the PMI information including G PMIs available to be used bythe first base station to send downlink data and G PMIs available to beused by a second base station to send downlink data; instruct thetransceiver to receive a channel quality request message for requestingchannel quality information from the first base station, the channelquality request message indicating that the mobile station is selected,by the first base station from among a plurality of candidate mobilestations, to perform multi-cell cooperation; calculate G2 channelquality indicators (CQIs) based on G² combinations of the G PMIs of thefirst base station and the G PMIs of the second base station; andinstruct the transceiver to report the channel quality information tothe first base station, wherein the channel quality information includesa best CQI and a first PMI corresponding to the first best CQI for thefirst base station, and a second best CQI and a second PMI correspondingto the second best CQI for the second base station.
 4. The mobilestation of claim 3, wherein the plurality of candidate mobile stationseach reports its channel quality information to the first base station.